Structural Compromise between High Hardness and Crack

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Structural Compromise between High Hardness and Crack Resistance in Aluminoborate Glasses Kristine F. Frederiksen, Kacper Januchta, Nerea Mascaraque, Randall E Youngman, Mathieu Bauchy, Sylwester J. Rzoska, Michal Bockowski, and Morten M. Smedskjaer J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b02905 • Publication Date (Web): 16 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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The Journal of Physical Chemistry

Structural Compromise between High Hardness and Crack Resistance in Aluminoborate Glasses Kristine F. Frederiksen1, Kacper Januchta1, Nerea Mascaraque1, Randall E. Youngman2, Mathieu Bauchy3, Sylwester J. Rzoska4, Michal Bockowski4, Morten M. Smedskjaer1,* 1

Department of Chemistry and Bioscience, Aalborg University, Aalborg, Denmark

2

Science and Technology Division, Corning Incorporated, Corning, USA

3

Department of Civil and Environmental Engineering, University of California, Los Angeles, USA

4

Institute of High-Pressure Physics, Polish Academy of Sciences, Warsaw, Poland

5

Department of Materials and Production, Aalborg University, Aalborg, Denmark

*Corresponding author. e-mail: [email protected]

ABSTRACT Alkali aluminoborate glasses have recently been shown to exhibit a very high threshold for indentation cracking compared to other bulk oxide glasses. However, to enable the use of these materials in engineering applications, there is a need to improve their hardness by tuning the chemical composition. In this study, we substitute alkaline earth for alkali network-modifying species at fixed aluminoborate base glass composition and correlate it with changes in structure, mechanical properties, and densification behavior. We find that the increase in field strength (i.e., the charge-to-size ratio) achieved by substituting the alkaline earth oxide from BaO to MgO manifests itself in a monotonic increase in several properties, such as atomic packing density, glass transition temperature, densification ability, indentation hardness, and crack resistance. Although the use of alkaline earth oxides as modifier enables higher hardness values (increasing from 2.0 GPa for Cs to 5.8 GPa for Mg), their crack resistance is generally lower than that of the corresponding alkali aluminoborate glasses. We discuss the origin of this compromise between hardness and crack resistance in terms of the ability of the glass networks to undergo structural transformations and self-adapt under stress. We show that the extent of volume densification scales linearly with the number of pressure-induced coordination number changes of B and Al.

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I. INTRODUCTION The development of new glassy materials with tailored properties could offer a practical solution to major global challenges within medicine, energy, and advanced communication devices.1 The inherent brittleness and poor crack resistance of oxide glasses are among the main limitations for enabling these novel applications. Impact and scratch events lead to formation of cracks, which amplify local tensile stresses, resulting in catastrophic failures.2 Therefore, increasing the hardness and crack resistance of glasses is critical for the development of scratch-resistant and mechanically durable glasses. All present oxide glasses suffer from low fracture toughness (Kc < ~1 MPa m½) and damage resistance.3 Moreover, there is often a conflict between hardness and damage resistance, as structural rearrangements through compositional tuning or post-treatment that tend to make glasses harder, typically also result in lower resistance to crack initiation and growth.4,5 That is, when the glass is able to undergo deformation easily, the local high stresses can be dissipated, thus avoiding cracking or vice versa.6 However, we note that notable exceptions exist, such as nitridation, chemical strengthening, and partial crystallization.3 The mechanical properties of glasses are governed by their atomic-scale structure, which, in turn, depends on the chemical composition. Both hardness and crack resistance, as typically determined using an instrumented indentation test, are governed by the sharp contact deformation mechanism, involving a combination of elasticity, densification, and isochoric shear flow in the zone beneath the loading contact area. The resistances of these processes are highly composition dependent, and thus govern hardness and crack resistance.7 Recently, we have discovered that alkali aluminoborate glasses exhibit high crack resistance.8-10 Particularly we found a 24Li2O−21Al2O3−55B2O3 (mol%) composition to exhibit the highest crack resistance (~31 N) ever reported for non-post-treated, melt-quenched oxide glasses. This has been ascribed to its highly adaptive network structure under stress. Unfortunately, these alkali aluminoborate glasses suffer from relatively low chemical durability and hardness, such as Vickers hardness of 4.1 GPa (measured at 9.8 N load) for the lithium aluminoborate glass. In this paper, we investigate whether it is possible to achieve a combination of high crack resistance and hardness in aluminoborate glasses by substituting alkali with alkaline earth modifier oxides. Due to their stronger modifier-oxygen bonds, alkaline-


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earth oxides should be candidates for higher hardness and moduli glasses,11,12 in addition to improved chemical durability.13 The substitution of alkali with alkaline earth modifiers in the aluminoborate networks will also improve the understanding of how chemical composition and structure influence the mechanical properties and cracking behavior of oxide glasses. In detail, we prepare a series of four alkaline earth aluminoborate glasses with nominal composition 25RO−20Al2O3−55B2O3 (R = Mg, Ca, Sr, Ba) to enable comparison with the similar alkali aluminoborate glasses from Ref.10. The concentrations of modifier oxide, alumina, and boria are fixed to properly understand the role of the modifier in aluminoborate networks. The modifiers cause changes in the aluminoborate network, as they serve as charge-balancing ions for the four-fold coordinated aluminum and boron tetrahedra, as well as create non-bridging oxygens (NBOs).14,15 The difference between the modifying cations can be quantified through the modifier field strength (FS), which is defined by Dietzel as:16 =

,

(1)

where zc and a are the charge of the cation and the summation of the ionic radii of the cation and anion, respectively.16 For example, it has been found that cations with higher FS promote the formation of NBOs in the structure, as the greater concentration of negative charge on NBOs compared to bridging oxygens (BOs) helps to stabilize the local coordination environment of the high-FS modifiers.17 The present study will provide additional knowledge about the role of FS on structure and mechanical properties of aluminoborate glasses, as alkaline earth cations exhibit higher FS than alkali cations. The atomic packing density of the glasses is calculated based on density measurements, while the coordination environments of boron and aluminum are obtained from 11B and 27Al magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy measurements, respectively. We correlate the structural differences with those of the mechanical properties, including hardness and crack resistance as determined from Vickers indentation. To further understand the glasses’ densification and deformation mechanisms, we compare the trends in mechanical properties with the tendency of the glasses to densify when subjected to isostatic compression at elevated temperature (hot compression).



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II. EXPERIMENTAL SECTION A. Sample Preparation To prepare the four 25RO−20Al2O3−55B2O3 (R = Mg, Ca, Sr, Ba) glasses, analytical reagent-grade raw materials (MgCO3, CaCO3, SrCO3, BaCO3, and Al2O3) were first dried in an oven at 105 °C for 24 h. H3BO3 was also used as raw material, but was not dried. The appropriate amounts were mixed and then melted in a Pt-Rh crucible at a temperature between 1100 and 1300 °C depending on the composition to ensure a homogeneous and bubble-free melt. Afterwards, the melts were quenched on a metal plate and annealed around their estimated glass-transition temperature (Tg) to relieve internal stresses. The exact Tg values were subsequently determined using differential scanning calorimetry (DSC) measurements at 10 K/min heating rate ((DSC 404 C, Netzsch) (Table 1). Afterwards the glasses were annealed at their measured Tg value for 2 h. The chemical compositions were analyzed by inductively coupled plasma optical emission spectroscopy (Table 1). The compositions are found to be within around ±2 mol% of the nominal ones, with the main deviation being due to volatilization of B2O3 during melting. The annealed samples were cut (7×7×2 mm3) and optically polished in ethanol using SiC adhesive discs with increasing grit size and finally a water-free diamond suspension on a polishing cloth in order to prevent surface hydration. Samples from each glass composition were then subjected to hot compression treatment, which was done in a nitrogen gas pressure chamber (1 GPa) for 1 h at their respective Tg. The furnace was afterwards cooled down to room temperature at a constant rate of 60 K/min, after which the system was decompressed at room temperature. The procedure is described in detail elsewhere.18

B. Solid State NMR Spectroscopy All as-made and compressed glasses were analyzed using

11

B and

27

Al MAS NMR. The glasses were

powdered and packed into 3.2 mm zirconia rotors, with sample spinning of 20 kHz.

27

Al NMR

measurements were made with a commercial spectrometer (Agilent DD2) using a 16.4 T narrow-bore superconducting magnet and a 3.2 mm MAS NMR probe (Agilent). 11B NMR measurements were conducted using a VNMRs commercial spectrometer with an 11.7 T wide-bore superconducting magnet and a 3.2 mm


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NMR probe (Varian T3). The data were collected at resonance frequencies of 182.34 and 160.34 MHz for 27

Al and

11

B NMR, respectively, using a 0.6 μs pulse-width (π/12 tip angle). The recycle delay between

acquisitions was 4 s, and the number of acquisitions was 400. Fitting of the MAS NMR spectra was done using DMFit.19 The CzSimple model was utilized for

27

Al NMR, thus accounting for the distribution in

quadropolar coupling constant. The “Q MAS 1/2” and Gaus/Lor functions were used to fit the three- and four-fold coordinated boron resonances, respectively.

C. Density Density (ρ) measurements for both as-made and compressed samples were done using Archimedes principle with ethanol as the immersion liquid. Each sample was weighed in air and in liquid ten times. Based on the permanent increase in density upon the hot compression treatment, we compute the plastic compressibility (β), which is determined from the initial density (ρinitial), the density after compression (ρafter), and the pressure (P) applied to the samples: =

after initial .

initial ∙

(2)

The density and NMR data are also used to calculate the atomic packing density (Cg), which describes the fraction of occupied volume and thus the packing efficiency of the glass. Cg is calculated as, ∑

= ∑ .

(3)

For the ith constituent with the formula AxBy, fi is the molar fraction, Mi is the molar mass, and = !

"# (%&#! + (&)! ) is the theoretical volume, where rA and rB are the ionic radii. It is assumed that the

constituent ions are spherical and the ionic radii are taken from Ref.20.

D. Indentation Hardness and cracking behavior of the samples were determined using micro-indentation. The measurements (Duramin 5, Struer A/S) were performed using a Vickers indenter (four-fold pyramid shaped diamond with an angle of 136°) at room temperature with relative humidity (46±8%). Each glass sample was indented 30 times at eight different loads P (0.025, 0.05, 0.1, 0.2, 0.3, 0.5, 1, and 2 kgf) for 15 s. The average half-lengths



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of the indent diagonals (a) were measured, and the number of cracks appearing at the corners of the indents were counted. The Vickers hardness (HV) was determined as

+ = 1.854 ,

(4)

while the crack resistance (CR) was determined as the load at which the crack probability is 50 %.21 Crack probability at a given load is calculated as the number cracks (zero to four) divided by the number of corners (four). CR and HV are listed in Table 1 for as-made and compressed glasses.

III. RESULTS AND DISCUSSION A. Boron and aluminum speciation The coordination number of boron can increase from three to four in the presence of modifier cations, such as alkali and alkaline earth.22-24 Addition of alumina typically results in a decreased fraction of four-fold coordinated boron, as the oxygens of the modifiers are used in the formation of aluminum tetrahedra instead. 22

The structural role of aluminum is complicated, as it can either act as a network former in four-fold

coordination or as a modifier in five- or six-fold coordination.22,23 This is controlled by the alumina/modifier ratio and presence of other network formers.22 Moreover, the pressure history affects the coordination state of both boron and aluminum, with compression generally leading to an increase in their coordination number due to the increased packing density.25,26 The obtained

11

B and

27

Al MAS NMR spectra of the present aluminoborate glasses demonstrate

significant differences in the short-range structures around B and Al as a function of the alkaline earth oxide (Fig 1). The boron atoms are predominantly present in three-fold coordination (BIII) at ambient pressure (Fig. 1a), with only a slight dependence on the FS of the modifier. On the other hand, the aluminum speciation is highly composition dependent (Fig. 1b). The concentration of five-fold aluminum (AlV) exceeds that of fourfold coordinated aluminum (AlIV) in the Mg-glass, while the concentration of AlIV exceeds that of higher coordinated Al (AlV and AlVI) in the remaining glasses in the series. Deconvolution of the MAS NMR spectra, the NMR parameters of which are provided in Table 2, yields additional quantitative information of the structural units in the network (Table 3). We further distinguish between trigonal boron units in non-ring


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(BIIInon-ring) and ring (BIIIring) sites, based on their respective isotropic chemical shift values.27 In addition, the fitting parameters used to deconvolute these

11

B MAS NMR data indicate low values of the quadrupolar

coupling asymmetry parameter (ηQ), on the order of 0.25 for most of the glasses, which is typical of symmetric BIII units having all bridging oxygen atoms.28 This indicates that any NBO on borate polyhedra is probably quite low, as asymmetric BIII units would exhibit larger ηQ values and make the 11B MAS NMR fitting much more complicated. The observed increase in the fractions of AlV and AlVI with increasing FS agrees with findings in previous studies.29 It has been suggested that higher FS modifier cations compete with the aluminum for shorter and stronger bonds to oxygen, which may force the aluminum to bond with a greater number of oxygens with longer bonds, thus increasing its coordination number.29 The previously studied alkali aluminoborate glasses exhibit higher concentrations of BIII and especially AlIV compared to the present alkaline earth aluminoborates with same Al/B ratio.10 Only the Li-glass contains a relatively high amount of AlV in the alkali-containing systems.9,10 Data obtained on alkali modified glasses8-10 suggests a polymerized network, consisting of boron and aluminum tetrahedra, thus indicating few to no NBOs in the structure. Due to the smaller concentration of four-fold coordinated aluminum atoms, that require charge-balancing modifier cations, in the present alkaline earth glasses, more NBOs are expected to be present in these glasses compared to the alkali versions. In spite of this implication, the 11B NMR data do not provide any evidence for NBO, as described above, and thus the higher number of NBO expected in these alkaline earth glasses is still near or below the detection limit using

11

B NMR

spectroscopy. Another structural difference is expected for the Mg-containing glass, where Mg2+ can exist in four-fold coordination as a network-forming cation, as well as in higher coordination as a modifier.30 Fewer network modifying ions are thus available for charge compensation of AlIV and BIV in this glass, reflected in the lower quantities of AlIV and BIV measured with NMR (Table 3), and resulting in a higher concentration of AlV and AlVI.31 In agreement with previous studies,25 four-fold coordinated boron and higher coordinated aluminum are more abundant in the compressed glasses. Previous studies on compressed B-containing glasses also showed that the fraction of BIII in superstructural units (i.e., ring BIII) decreases upon compression, resulting in a



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higher fraction of non-ring BIII.32 This is also the case for these alkaline earth aluminoborate glasses, except for the Mg glass, with compression leading to a reduction in BIIIring by 3 to 13%, depending on the modifier field strength (Table 3). However, we note that unlike the determination of the total fractions of BIII and BIV, the quantification of ring and non-ring BIII units is relatively uncertain and therefore we do not discuss the trends in these further herein. The modifier field strength affects the extent of other pressure-induced changes, as oxygen linkages with relatively high negative charges such as AlV-O-BIV and AlVI-O-BIV will be more abundant with increasing pressure. Higher FS cations are more effective in stabilizing such linkages, thus promoting the formation of higher coordinated species.29 Previous studies of aluminosilicate33 and borosilicate glasses34 have suggested that the amount of NBOs in the structure is important for the increasing coordination of both Al and B upon compression, as they are to a large extent expected to be formed at the expense of NBOs in the structure.29 This aligns with the present findings, as an increase in FS yields more of the higher coordinated Al and B in the structure (Table 2). However, based on

11

B NMR data, the NBO

contents in the present glasses are likely too small to account for these pressure-induced changes, implying that other mechanisms, e.g., involving formation of oxygen triclusters, may be important.25 We also note that in the as-made Mg-glass, Al acts mostly as a modifier given the high abundance of AlV. Interestingly, both AlIV and modifying AlV sites are consumed during densification (Table 3). Hence, the structural mechanism facilitating densification in the Mg-glass is not only from AlIV to AlV and AlIV to AlVI, but also AlV to AlVI, yielding a relatively large content of AlVI in the structure of the Mg-glass after compression. For the remaining glasses in the series, we note that the pressure-induced decrease in the fraction of AlIV units roughly corresponds to the increase in that of BIV units. As previously discussed,9,35 this interaction between B and Al is due to the need for modifier cations to compensate the partial negative charges that develop on bridging oxygens in BIV units. This pressure-induced effect effectively reduces the charge compensation available for AlIV, thus promoting the formation of higher-coordinated Al species. To quantify this interaction, the atomic concentration of AlIV and BIV are computed from the MAS NMR data (Table 3) and the fractions of the corresponding oxides (Table 1), for both the as-made and compressed samples. In a [BIV] vs. [AlIV] scatter plot (Fig. 2a), the two data points for each glass composition indicate the relative rates of [AlIV] removal and [BIV] formation upon compression. For the alkaline earth aluminoborates studied


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herein (with the exception of Mg-glass), for each mol of created BIV, approximately one mol of AlIV is consumed, as reflected by a slope of around –1. For comparison, we also include the data for Naaluminoborate, Li-aluminoborate, and Ca-aluminoborosilicate glasses.8,9,35 As seen in Fig. 2a, not all compositions exhibit the slope equal to –1, which would be indicative of a one-to-one BIV for AlIV replacement. To understand this, the obtained slopes are plotted as a function of the molar [Al2O3]/[B2O3] ratio in the glasses (Fig. 2b). The glasses with steep slopes, such as some of the Ca-aluminoborosilicates,35 have relatively large [Al2O3]/[B2O3] ratios, while glasses with much smaller slopes have smaller [Al2O3]/[B2O3] ratios. In other words, when the concentration of either Al2O3 or B2O3 is small, the probability of replacing one AlIV with exactly one BIV unit is low, with other pressure-induced transformation occurring (e.g., formation of oxygen triclusters). The present Mg-glass has an intermediary value of [Al2O3]/[B2O3], but its low fraction of AlIV in the asmade state (~30%) reduces the probability of efficient pressure-induced AlIV conversion. On the other hand, when the concentrations of [Al2O3] and [B2O3] are both significant and close to each other, the favored mechanism occurring during compression is the formation of BIV units at the expense of AlIV units, as discussed above. The Al-cations which lost their charge-balancing modifier cations during compression would then enter a modifying role themselves, reflected by the observed increase in the fraction of five- and six-coordinated states.

B. Packing density and glass transition temperature The calculated atomic packing density increases with increasing FS for both the alkali10 and alkaline earth aluminoborate glasses (Fig. 3a). This trend suggests that the size of the ions is not the only determining factor for packing efficiency, as the ionic radius of Ba is significantly larger than that of Li, while their packing densities are similar. The modifiers with a higher positive charge and thus a higher FS are more prone to attract the O-atoms from the surroundings, and thus create a more efficient packing of the structure. This has also been found in previous studies, in which it was also suggested that the higher coordination states of Al improve the packing efficiency.36 Increasing the modifier field strength also increases the glass transition temperature (Tg) (Fig. 3b). The structural studies presented above reveal that the amounts of BIV 9 ACS Paragon Plus Environment


and especially AlIV decrease with FS, implying less atomic constraints and thus smaller network connectivity. This should yield a decrease in Tg.37 However, network rigidity is also affected by the strength of the modifier-oxygen constraints.38 Higher FS cations create stronger bonds to oxygen, correlating with the observed trend in Tg in this study. The bond-strength of Li-O resembles that of Mg-O,12 but the Tg of the Liglass is much lower than that of the Mg-glass. This is attributed to the lower packing density of the Li-glass, giving rise to a less rigid structure. Moreover, the likely formation of network-forming Mg tetrahedra, manifested in the lower fraction of AlIV and BIV polyhedra, results in a more polymerized network and thus higher Tg. We next quantify the permanent change in density upon hot compression using plastic compressibility (β) (Fig. 4). There is no systematic relation between FS and β among the two series, as the alkali series exhibits a local minimum and the alkaline-earth series exhibits a positive correlation. The local minimum in FS vs. β indicates the existence of two competing mechanisms. It has been suggested that the low-FS glasses are dominated by medium-range order reorganization, as the low packing efficiency leaves room for densification of the network without significant changes to the coordination numbers of the network-forming cations.10 The mechanism controlling the densification of the higher-FS alkali aluminoborates is suggested to be associated with the ability of the cations to charge-balance more boron and aluminum tetrahedra at high pressure, as smaller cations occupy less space around these tetrahedral.10 Previous work on Li and Na containing aluminoborates has shown that glasses with self-adapting networks are enabling more permanent densification for higher FS glasses.8,9 That is, it is suggested that a smaller resistance towards changes in the connectivity of the glass causes the increase in β with increasing FS for the alkali series10 and for the present alkaline earth series. The pressure-induced changes in connectivity of the alkaline earth aluminoborate glasses can, as a first approximation, be quantified by the rates of AlIV removal and BIV formation relative to the applied pressure (d[AlIV]/dP and d[BIV]/dP, respectively). For compositions with d[AlIV]/d[BIV] close to unity, it might be sufficient to consider the change in only one of the species, as the two are dependent on each other, as discussed in Section III.A. However, as illustrated in Fig. 2a, several aluminoborate and aluminoborosilicate glasses do not follow this simple one-to-one relation. Therefore, we compute the sum of both connectivity


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changes (d([BIV]-[AlIV])/dP) and correlate it with the glasses’ ability to densify when subjected to pressure (Fig. 5). Note that we use -d[AlIV] as AlIV units are removed upon compression, while we use +d[BIV] as BIV units are formed. In addition to the present alkaline earth aluminoborate glasses, we include high-pressure structural data for Na-aluminoborate, Li-aluminoborate, Ca-aluminoborosilicate, Na,Ca-borosilicate, Na,Caborate, and Na-aluminosilicate glasses8,9,35,39-41 to investigate the universality of the trend. The increase in the coordination numbers of both Al and B indeed appears to facilitate the hightemperature densification, as we observe a positive linear relation between β and d([BIV]-[AlIV])/dP (Fig. 5). We note that the glasses with very small changes in the coordination numbers of the network-forming cations (e.g., the Na-aluminosilicate glasses) are also able to densify (~2% density increase at 1 GPa hot compression). This suggests that there are other structural mechanisms facilitating densification in addition to the B- and Al-coordination changes.42 Oxide glasses generally have relatively low packing efficiency, which enables them to reduce the free volume in their networks by bond angle alterations, with limited changes in the connectivity. Densification may also involve changes in intermediate-range structure,43 as seen here in a compression-induced reduction in borate superstructural units, where connectivity between network polyhedra is altered, but short-range structure (i.e., cation coordination number) is preserved. Hence, the intercept with the y-axis in Fig. 5 could be interpreted as the “baseline” compressibility associated with any structural rearrangements that do not lead to coordination changes. Such approximation assumes that the range of oxide glasses included in Fig. 5 have similar baseline compressibility (~0.02 GPa-1), which may not be completely accurate for glasses with different atomic packing densities. It is important to emphasize that the relation found in Fig. 5 is only valid for glasses with networks consisting of B and/or Al. B- and Alcontaining silicate glasses are included as well, since Si is known to change its coordination at much higher pressures than the range of pressure used in this study.25 Hence, it is not necessary to consider the chemical environment change of Si.

C. Hardness and crack resistance Hardness represents the resistance to elastoplastic deformation. Increasing the FS of the modifying cation has a positive effect on hardness (Fig. 6) and, as such, substitution of alkali for alkaline earth ions increases the



glass hardness. The introduction of alkaline earth cations to the crack resistant aluminoborate glasses thus improves the hardness as desired. Hardness of all the compositions increases further when subjected to 1 GPa isostatic compression, in agreement with what is found in previous studies of hot compression of glass.25 These previous studies suggest that a larger resistance towards densification beneath the indentation tip is responsible for the increased hardness. Crack resistance (CR) of glasses depends on their deformation mechanism.3 Glass compositions, which are prone to densification, have been reported to be more crack-resistant,44 as it reduces the driving force (residual stresses) for cracking, although sufficiently high toughness could also impede crack formation. The two series of aluminoborate glasses exhibit different dependence of CR on modifier field strength (Fig. 7). The alkali series exhibit a local minimum in CR vs. FS, whereas the crack resistance of the alkaline-earth series monotonically increases with FS. Overall, the alkali-modified glasses are more crack-resistant than the alkaline earth glasses. Assuming that the glasses feature similar toughness values, this indicates that the driving force for cracking is larger in the alkaline earth series. More NBOs in the structure have been associated with a higher extent of shear flow, which will develop large residual stresses.45 Based on the structural data, we inferred that more NBOs are present in the alkaline earth compared to the alkali glasses, although the total content is low, which might explain their lower crack resistance. The crack resistance decreases upon compression, as previously reported.25 This is due to less available space into which the glass can densify prior to cracking, resulting in increased residual stress, which is the driving force for crack initiation. Similarly to the findings in Ref.9, we here find crack resistance to increase with the glasses’ ability to densify under pressure and increase their network connectivity (although the ability of the Mg-glass to change connectivity is obscured by its low initial AlIV content). In other words, more self-adaptive glasses exhibit higher crack resistance values. However, the absolute values of atomic self-adaptivity (data not shown) for the present glasses do not agree with those found previously,9 suggesting that additional work is needed to develop a predictive model for indentation crack resistance. Finally, we also note that while crack resistance of the Mg-glass is ~30% lower than that of the Li-glass (Fig. 7), its hardness is correspondingly


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~40% higher (Fig. 6). This presents an opportunity to structurally tailor the mechanical properties of aluminoborate glasses by selecting the modifier depending on the application.

IV. CONCLUSION The structure, mechanical properties, and densification behavior of alkaline-earth modified aluminoborate glasses are examined and compared to their alkali counterparts. Substituting alkali for alkaline earth oxides gives higher field strength modifiers, which yields higher values of packing density, glass transition temperature, and hardness, as the high-FS modifiers form stronger modifier-oxygen bonds. Crack resistance exhibits different trends for the two series, as the crack resistance first decreases and then increases with increasing field strength for the alkali series, while it monotonically increases with field strength for alkaline earth series. This agrees with the trends observed for plastic compressibility, as glasses more prone to densification exhibit higher crack resistance. When subjected to pressure, four-fold coordinated boron units are created at the expense of four-fold coordinated aluminum units. This mechanism facilitates densification, thus improving crack resistance, and the ability of different B- and/or Al-containing oxide glasses to densify scales linearly with the amount of such structural rearrangements. This study has also shown that a compromise between high hardness and crack resistance exists for aluminoborate glasses. The structural features that make the glasses harder also inhibit the glasses to deform through densification, thus effectively increasing the driving force for indentation cracking. The best compromise is found for the present magnesium aluminoborate glass that features relatively high crack resistance (above 2 kgf) and hardness (5.8 GPa) and it could thus find interesting industrial applications.

AUTHOR INFORMATION Corresponding Author. *E-mail: [email protected] Notes. The authors declare no competing financial interest.



ACKNOWLEDGEMENTS This work was supported by VILLUM FONDEN under research grant no. 13253. Partial financial support was offered by the National Science Foundation under Grant No. 1562066.


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25. Kapoor, S.; Wondraczek, L.; Smedskjaer, M. M. Pressure-induced densification of oxide glasses at the glass transition. Front. Mater. 2017, 4, 1. 26. Wondraczek, L.; Behrens, H. Molar volume, excess enthalpy, and Prigogine-Defay ratio of some silicate glasses with different (P,T) histories. J. Chem. Phys. 2007, 127, 154503. 27. Youngman, R. E.; Zwanziger, J. W. Network modification in potassium borate glasses: Structural studies with NMR and Raman Spectroscopies. J. Phys. Chem. 1996, 100, 16720-16728. 28. Kroeker, S.; Stebbins, J. F. Three-coordinated boron-11 chemical shifts in borates. Inorg. Chem. 2001, 40, 6239-6246. 29. Bista, S.; Morin, E. I.; Stebbins, J. F. Response of complex networks to compression: Ca, La, and Y aluminoborosilicate glasses formed from liquids at 1 to 3 GPa pressures. J. Chem. Phys. 2016, 144, 044502. 30. Pedone, A.; Malavasi, G.; Menziani, M. C.; Segre, U.; Cormack, A. N. Role of magnesium in soda-lime glasses: Insight into structural, transport, and mechanical properties through computer simulations. J. Phys. Chem. C 2008, 112, 11034-11041. 31. Watts, S. J.; Hill, R.; O’Donnell, M. D.; Law, R. Influence of magnesia on the structure and properties of bioactive glasses. J. Non-Cryst. Solids 2010, 356, 517-524. 32. Svenson, M. N.; Youngman, R. E.; Yue, Y. Z.; Rzoska, S. J.; Bockowski, M.; Jensen, L. R.; Smedskjaer, M. M. Volume and structural relaxation in compressed sodium borate glass. Phys. Chem. Chem. Phys. 2016, 18, 29879-29891. 33. Lee, S. K.; Yi, Y. S.; Cody, G. D.; Mibe, K.; Fei, Y.; Mysen, B. O. Effect of network polymerization on the pressure-induced structural changes in sodium aluminosilicate glasses and melts: 27Al and 17O solidstate NMR study. J. Phys. Chem. C 2012, 116, 2183-2191. 34. Du, L. S.; Allwardt, J. R.; Schmidt, B. C.; Stebbins, J. F. Pressure-induced structural changes in a borosilicate glass-forming liquid: Boron coordination, non-bridging oxygens, and network ordering. J. Non-Cryst. Solids 2004, 337, 196-200.



35. Bista, S.; Stebbins, J. F.; Wu, J.; Gross, T. M. Structural changes in calcium aluminoborosilicate glasses recovered from pressures of 1.5 to 3GPa: Interactions of two network species with coordination number increases. J. Non-Cryst. Solids 2017, 478, 50-57. 36. Kaneko, S.; Tokuda, Y.; Masai, H. Additive effects of rare-earth ions in sodium aluminoborate glasses using 23Na and 27Al magic angle spinning nuclear magnetic resonance. New J. Glass Ceram. 2017, 7, 5876. 37. Mauro, J. C.; Gupta, P. K.; Loucks, R. J. Composition dependence of glass transition temperature and fragility. II. A topological model of alkali borate liquids. J. Chem. Phys. 2009, 130, 234503. 38. Rodrigues, B. P.; Wondraczek, L. Cationic constraint effects in metaphosphate glasses. J. Chem. Phys. 2014, 140, 214501. 39. Svenson, M. N.; Bechgaard, T. K.; Fuglsang, S. D.; Pedersen, R. H.; Tjell, A. Ø.; Østergaard, M. B.; Youngman, R. E.; Mauro, J. C.; Rzoska, S. J.; Bockowski, M.; Smedskjaer, M. M. Compositionstructure-property relations of compressed borosilicate glasses. Phys. Rev. Appl. 2014, 2, 024006. 40. Smedskjaer, M. M.; Youngman, R. E.; Stripe, S.; Potuzak, M.; Bauer, U.; Deubener, J.; Behrens, H.; Mauro, J. C.; Yue, Y. Z. Irreversibility of pressure induced boron speciation change in glass. Sci. Rep. 2014, 4, 3770. 41. Bechgaard, T. K.; Goel, A.; Youngman, R. E.; Mauro, J. C.; Rzoska, S. J.; Bockowski, M.; Jensen, L. R.; Smedskjaer, M. M. Structure and mechanical properties of compressed sodium aluminosilicate glasses: Role of non-bridging oxygens. J. Non-Cryst. Solids 2016, 441, 49-57. 42. Wu, J.; Deubener, J.; Stebbins, J. F.; Grygarova, L.; Behrens, H.; Wondraczek, L.; Yue, Y. Z. Structural response of a highly viscous aluminoborosilicate melt to isotropic and anisotropic compressions. J. Chem. Phys. 2009, 131, 104504. 43. Smedskjaer, M. M.; Bauchy, M.; Mauro, J. C.; Rzoska, S. J.; Bockowski, M. Unique effects of thermal and pressure histories on glass hardness: Structural and topological origin. J. Chem. Phys. 2015, 143, 164505. 44. Kato, Y.; Yamazaki, H.; Kubo, Y.; Yoshida, S.; Matsuoka, J.; Akai, T. Effect of B2O3 content on crack initiation under Vickers indentation test. J. Ceram. Soc. Jpn. 2010, 118, 792-798.


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45. Gross, T. M. Deformation and cracking behavior of glasses indented with diamond tips of various sharpness. J. Non-Cryst. Solids 2012, 358, 3445-3452.



Page 20 of 30

TABLES Table 1. Analyzed chemical composition, glass transition temperature (Tg), Vickers hardness (HV) measured at 2 kgf, and crack resistance (CR) of the as-prepared and compressed alkaline earth aluminoborate glasses. The errors of the chemical compositions, Tg, HV, and CR do not exceed ±0.1 mol, ±2 °C, ±0.1 GPa, and ±0.2 kgf, respectively. Glass ID R=Mg R=Ca R=Sr R=Ba

Chemical composition (mol%) RO Al2O3 B2O3 26.3 19.7 54.0 25.8 21.5 52.7 27.0 19.6 53.4 27.5 19.9 52.6

Tg (°C) 636 615 590 554

HV (GPa) as-made 1 GPa 5.8 7.7 5.0 7.0 4.9 6.1 4.5 5.5


CR (kgf) as-made 1 GPa 2.2 0.7 1.1 0.5 1.1 0.3 0.9 0.2

Page 21 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. NMR parameters used to fit the

27

Al and 11B MAS NMR spectra: isotropic chemical shift (δCS),

quadrupolar coupling constant (CQ), and quadrupolar coupling asymmetry parameter (ηQ). Uncertainties do not exceed ±0.2 ppm for δCS, ±0.2 MHz for CQ, and ±0.05 for ηQ. BIV peaks were fit using a combination of Gaussian and Lorentzian lineshapes, as described in the text, and thus do not provide CQ and ηQ information, though both parameters are typically very small. Al sites

12 3

Glass ID

B sites

12

333 4567

12 3

333 4686567

43

δCS

CQ

δCS

CQ

δCS

CQ

δCS

CQ

ηQ

δCS

CQ

ηQ

δCS

(ppm)

(MHz)

(ppm)

(MHz)

(ppm)

(MHz)

(ppm)

(MHz)

(-)

(ppm)

(MHz)

(-)

(ppm)

R=Mg (as-made)

56

3.0

31

5.4

4

3.8

18

2.7

0.24

15

2.5

0.13

0

R=Mg (1 GPa)

56

2.4

31

5.4

4

4.3

18

2.7

0.28

14

2.6

0.03

0

R=Ca (as-made)

58

3.6

32

5.2

4

3.4

18

2.7

0.26

15

2.6

0.20

0

R=Ca (1 GPa)

59

3.6

32

5.5

4

3.8

18

2.7

0.26

15

2.6

0.12

0

R=Sr (as-made)

59

4.9

32

5.3

2

2.1

18

2.7

0.26

15

2.6

0.20

1

R=Sr (1 GPa)

59

3.7

32

5.5

3

1.9

18

2.7

0.26

15

2.6

0.22

1

60

4.0

31

5.5

2

3.2

18

2.7

0.25

15

2.6

0.24

1

61

4.0

33

5.6

5

2.4

18

2.7

0.25

15

2.6

0.24

1

R=Ba (as-made) R=Ba (1 GPa)



Page 22 of 30

Table 3. Fractions of three- and four-fold coordinated boron and four-, five-, and six-fold coordinated aluminum in both as-prepared and compressed glasses. Trigonal boron units are further divided into non-ring (BIIInon-ring) and ring (BIIIring) sites. The fractions for boron and aluminum are determined through deconvolution of the 11B and 27Al MAS NMR spectra, respectively. The uncertainties are approximately ±1 at%. Glass ID R=Mg (as-made) R=Mg (1 GPa) R=Ca (as-made) R=Ca (1 GPa) R=Sr (as-made) R=Sr (1 GPa) R=Ba (as-made) R=Ba (1 GPa)

Atomic fraction (%) BIIItotal BIV AlIV

BIIIring

BIIInon-ring

AlV

AlVI

56

32

87

13

30

57

13

56

21

77

23

20

54

27

64

24

87

13

57

37

6

59

18

78

22

38

46

15

66

18

83

17

77

21

2

59

19

77

23

53

38

9

71

13

84

16

87

11

1

58

20

78

22

68

27

5


Page 23 of 30

FIGURES Figure 1. (a) 11B and (b) 27Al MAS NMR spectra of the as-prepared (solid lines) and compressed (dashed lines) aluminoborate glasses with different alkaline earth oxides. (a)

III

B

IV

Intensity (a.u.)

B

25

20

15

10 11

(b)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80

0

-5

-10

B MAS NMR shift (ppm)

Mg Ca Sr Ba solid: as-prepared dashed: compressed

Al

5

Mg Ca Sr Ba solid: as-prepared dashed: compressed

Al

V

Al

VI

IV

60

40

20

0

27

Al MAS NMR shift (ppm)


-20

-15


Figure 2. (a) Pressure-induced changes in the fraction of four-fold coordinated Al ([AlIV]) and B ([BIV]) for Na-aluminoborates,8 Li-aluminoborate,9 alkaline earth (AE) aluminoborates (this study), and Caaluminoborosilicates.29 The data points for each composition (connected by solid line) represent the fractions for the as-made and hot compressed samples. The number represents the slope of the [AlIV] vs. [BIV] relation, as explained in the text. (b) The relative rates of [AlIV] removal and [BIV] formation (d[AlIV]/d[BIV]) plotted as a function of the ratio of the molar fractions of Al2O3 and B2O3 ([Al2O3]/[B2O3]) for the same glasses.

(a)

60

Na-aluminoborates Li-aluminoborate AE-aluminoborates Ca-aluminoborosilicates

-1.7

50

[AlIV] (mol%)

-0.9

40 -1.2

30 20

-0.4

-1.4 -3.6

increasing P

-1.9 -1.2

-1.2

-0.8

-1.1

-0.4

10 -0.3

0 0

10

20

30

40

50

60

IV

[B ] (mol%)

(b) 0

Na-aluminoborates Li-aluminoborate AE-aluminoborates Ca-aluminoborosilicates

increasing FS

-1

d[AlIV]/d[BIV] (-)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 30

-2

increasing [Al2O3]

-3 decreasing [B2O3]

-4 0

1

2

[Al2O3]/[B2O3] (-)


3

Page 25 of 30

Figure 3. (a) Atomic packing density Cg and (b) glass transition temperature Tg as a function of the modifier field strength for the aluminoborate glasses. In (a), closed symbols represent as-prepared glasses, while open symbols represent compressed glasses. Data for the alkali series are obtained from Ref.10. The lines are guides for the eye. The errors in Cg and Tg are smaller than the size of the symbols. (a) 0.61

Alkaline earth Alkali

Compressed

0.58

Cg (-)

As-prepared Mg

0.55

Ca

Sr Ba Li

0.52 Na K Rb

0.49

Cs

0.46 0.2

0.3

0.4

Field Strength (

(b) 675

0.5

2

0.1

Å

)

Alkaline earth Alkali Ca

600

Mg

Sr Ba

o

Tg ( C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


525

Li

450

Na K Cs Rb

375 0.1

0.2

0.3

0.4 -2

Field strength (Å )


0.5


Figure 4. Dependence of plastic compressibility (β) on the modifier field strength for the aluminoborate glasses. Plastic compressibility is calculated based on the density data using Eq. (2). Data for the alkali series

-1

Plastic Compressiblity (GPa )

are obtained from Ref.10. The lines are guides for the eye. The error in β is around ±0.002 GPa-1.

0.08

Li Mg Ca

0.07

Cs Sr

0.06

Na Ba

Rb

0.05


K

0.2

0.3

Field Strength (

2

0.1

Å

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 30

)


0.4

0.5

Page 27 of 30

Figure 5. Plastic compressibility (β) as a function of the sum of the pressure-induced AlIV-removal and BIVformation (d([BIV]-[AlIV])/dP) for the Na-aluminoborates,8 Li-aluminoborate,9 alkaline earth (AE) aluminoborates (this study), Ca-aluminoborosilicates,29 Na,Ca-borosilicate,39 Na,Ca-borate,40 and Naaluminosilicates glasses.41 The dashed line is a guide for the eye. The error in β is around ±0.002 GPa-1.

0.08

0.06

β (1/GPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.04

0.02 Na-aluminoborates Li-aluminoborate AE-aluminoborates

0.00 0.00

0.05 IV

0.10

Ca-aluminoborosilicates Na,Ca-borosilicate Na,Ca-borate Na-aluminosilicates

0.15

IV

d([B ]-[Al ])/dP (mol%/GPa)


0.20


Figure 6. Vickers hardness HV as a function of the modifier field strength for the aluminoborate glasses. Closed symbols represent as-prepared glasses, while open symbols represent compressed glasses. Data for the alkali series are obtained from Ref.10. The lines are guides for the eye. The error in HV is around ±0.1 GPa. 8

Hardness (GPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 30

Compressed


6

As-prepared Mg Sr

Ca

Ba

4 Li Na K

2

Rb Cs

0.1

0.2

0.3

0.4 -2

Field Strength (Å )


0.5

Page 29 of 30

Figure 7. Crack resistance as a function of the modifier field strength for the aluminoborate glasses. Closed symbols represent as-prepared glasses, while open symbols represent compressed glasses. Data for the alkali series are obtained from Ref.10. The lines are guides for the eye. The error in crack resistance is around ±0.2 kgf. Alkaline earth Alkali

Rb

3

Crack resistance (kgf)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Li Cs

As-prepared

K

2

Mg Na

1

Sr

Ca

Compressed

Ba

0 0.1

0.2

0.3

0.4 -2

Field Strength (Å )


0.5


TOC GRAPHIC 8

Cs Rb K

Li

Na

Ba Sr

Mg

Ca

6 2 4 1 2

filled symbols: hardness open symbols: crack resistance

0.1

0.2

0.3

0.4

Crack Resistance (kgf)

3

Hardness (GPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0

0.5 -2

Modifier Field Strength (Å )


Page 30 of 30

Structural Compromise between High Hardness and Crack

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